Semiconductor nanocrystal quantum dots (NQDs) are desirable fluorophores based on their unique particle-size-tunable optical properties, i.e., efficient and broadband absorption and efficient and narrow-band emission. Further, compared to alternative fluorophores, such as organic dyes, NQDs are characterized by significantly enhanced photostabilility. Despite these desirable characteristics, NQD optical properties may be frustratingly sensitive to their surface chemistry and chemical environment. For example, coordinating organic ligands are used to passivate the NQD surface during growth, and are retained following preparation. These coordinating ligands are strong contributors to bulk NQD optical properties such as quantum yields (QYs) in emission; however, the ligands tend to be labile and can become uncoordinated from the NQD surface, and can be damaged by exposure to the light sources used for NQD photoexcitation. Ligand loss through physical separation or photochemistry results in uncontrolled changes in QYs and, in the case of irreversible and complete loss, in permanent “darkening” or photobleaching. In addition, some ligands may be incompatible with certain solvents and systems, thus limiting the uses of a particular NQD.
Furthermore, NQDs are characterized by significant fluorescence intermittency, or “blinking,” at the single NQD level. Without wishing to be limited by theory, blinking is generally considered to arise from an NQD charging process in which an electron (or a hole) is temporarily lost to the surrounding matrix (for example, via Auger ejection or charge tunneling) or captured to surface-related trap states. NQD emission turns “off” when the NQD is charged and turns “on” again when NQD charge neutrality is regained. Blinking is unacceptable for such potential NQD applications as single-photon light sources for quantum informatics and biolabels for real-time monitoring of single biomolecules. Previous attempts to address blinking include the use of charge mediators such as short-chain thiols on the NQD surface. This approach provided at best only a partial, short-term solution however, and encountered such problems as dependence on pH, concentration, lighting conditions, and the NQDs were further incompatible with a number of applications.
It is known that addition of an inorganic shell of a semiconductor material having a higher bandgap can generally enhance QYs and improve stability. See, for example, Hines, M. A.; Guyot-Sionnest, P. J. Phys. Chem. 1996, v. 100, pp. 468-471. However, the optical properties of previously disclosed core/shell and core/multishell NQDs remain susceptible to blinking, photobleaching and ligand issues. A need exists, therefore, for NQDs which have increased stability, and decreased fluorescence intermittency and photobleaching.